[Bug 1662495] [NEW] GCC 5 optimizer produces incorrect code

[Egor Suvorov]

Public bug reported:

Inspired by the following blog post:
http://codeforces.com/blog/entry/50291

Attached file is a snippet of code which, when compiled with `g++ a.cpp`
produces an executable `a.out`, which runs correctly. But when compiled
with `g++ a.cpp -O1` the resulting executable crashes in line 10
(according to GDB). Looks like array out-of-bounds error happens despite
corresponding check being present in the code.

We was able to reproduce it on two independent installations of Ubuntu
16.04, compiler version is `gcc version 5.4.0 20160609 (Ubuntu
5.4.0-6ubuntu1~16.04.4)` (looks like it's the most recent package
available as of February 7, 2017).

Steps to reproduce:
1. Download `a.cpp` attached.
2. Run `g++ a.cpp -O1` and ensure that it finished without any errors.
3. You can optionally run `g++ a.cpp -O1 -Wall -Wextra -Werror -pedantic` and ensure that this command finishes without any errors as well.
4. Run `./a.out`.

Expected behavior: program finishes correctly.

Real behavior: program crashes with segmentation fault.

** Affects: gcc-5 (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "a.cpp"
   https://bugs.launchpad.net/bugs/1662495/+attachment/4814685/+files/a.cpp

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https://bugs.launchpad.net/bugs/1662495

Title:
  GCC 5 optimizer produces incorrect code

Status in gcc-5 package in Ubuntu:
  New

Bug description:
  Inspired by the following blog post:
  http://codeforces.com/blog/entry/50291

  Attached file is a snippet of code which, when compiled with `g++
  a.cpp` produces an executable `a.out`, which runs correctly. But when
  compiled with `g++ a.cpp -O1` the resulting executable crashes in line
  10 (according to GDB). Looks like array out-of-bounds error happens
  despite corresponding check being present in the code.

  We was able to reproduce it on two independent installations of Ubuntu
  16.04, compiler version is `gcc version 5.4.0 20160609 (Ubuntu
  5.4.0-6ubuntu1~16.04.4)` (looks like it's the most recent package
  available as of February 7, 2017).

  Steps to reproduce:
  1. Download `a.cpp` attached.
  2. Run `g++ a.cpp -O1` and ensure that it finished without any errors.
  3. You can optionally run `g++ a.cpp -O1 -Wall -Wextra -Werror -pedantic` and ensure that this command finishes without any errors as well.
  4. Run `./a.out`.

  Expected behavior: program finishes correctly.

  Real behavior: program crashes with segmentation fault.

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